I'll carry a PPK/S in a pocket holster as a back up, but when I carry it as a primary I use a Wild Bills Concealment Covert Carry IWB holster. The same holster works fine with the PPK and the PP, although I think they make a specific holster for each.
Covert Carry
The aft mounted clip helps hold the butt in close you your body, makes the holster slightly thinner over all, and spreads the load and pressure over larger surface area improving comfort.
They are well made and hold up very well over time with heavy weight leather and sticking with a double thickness mouth with a plastic insert between the two layers of leather.
The neutral finish is also slightly porous so with a coat of shoe polish of your chosen color you can make them pretty much any color you want in about 10 minutes.
I've used one for daily carry with a Kimber Ultra Carry for 6 years now and while there is some darkening from sweat on the body side of the holster, it's held up extremely well, still retinas the pistol well, and still stays open in the waistband if you draw the pistol, so it can be re-holstered without removing the holster from your waist band.
Holster for PPK/S:
Six year old Holster for Ultra Carry: